<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <div id="app">
    <button @click="addLi">新增arr</button>
    <ul ref="oul">
      <li v-for="item in arr" :key="item">
        {{item}}
      </li>
    </ul>
  </div>
  <script src="./vue.js"></script>
  <script>

    const vm = new Vue({
      el: '#app',
      data: {
        arr: [1,2,3,4]
      },
      methods: {
        addLi(){
          this.arr.push(this.arr.length+1);
          
          // 观察者 观察时  dom是否更新
          this.$nextTick(() => {
            console.log(this.$refs.oul.children, 222);
            this.$refs.oul.lastChild.style.background='red';
          })
          console.log(this.$refs.oul.children, 111);
        }
      }
    })
  </script>
</body>

</html>